Light modulator including a collimator comprising an interference filter

ABSTRACT

A collimator is formed of a stack of layers (11) of different thicknesses in a way analogous to a low-pass interference filter. When illuminated by narrow-band light of a predetermined wavelength just within the pass band of the filter, the collimator preferentially transmits light incident within a predetermined angular range, usually near-normal. The collimator is especially useful with photoluminescent liquid-crystal display, having phosphor emitters (17), because the collimator layers can simply be deposited on one face of the liquid-crystal modulator (1).

This invention relates to an improved collimator, particularly for usewith a liquid-crystal display system of the type using aphotoluminescent screen, such as are discussed, for instance, in WO95/27920 (Crossland et al).

Means for producing collimated light suitable for use withliquid-crystal display cells are known. For example, IBM TechnicalDisclosure Bulletin (1987) discloses at page 4838 a system involving alight guide plate and a lens plate with a plurality of moulded lensletsplaced in front of the plate. The moulded lenslets collimate the lightpassing through holes in the light guide plate. A similar arrangement isdisclosed in European patent application No. 529 832 (RockwellInternational). In that case, phosphors are used to provide small spotsof light to be focused by a lens array.

Collimators can be desirable in liquid-crystal displays for variousreasons, one being that when a diffuse light source is used the lightpassing through a single cell, not being collimated, will spread andcover a substantial area of the photoluminescent screen, leading tocrosstalk. This will impair the resolution of the screen, and is ofparticular significance for LCDs using a photoluminescent screen, wherein colour displays it may result in a phosphor of the wrong colour beingilluminated. For the standard kinds of liquid-crystal cells increasingthe collimation of the backlighting has the disadvantageous effect ofreducing the angle of view of the display, though the display isbrighter in the collimation direction. The light from the display thushas to be spread out again using, for instance, a diffuser plate. Inphotoluminescent LCDs (PLLCDs), however, the phosphor on the frontscreen performs the diffusing Function to great advantage, as explainedin WO 95/27920. However, prior art collimators are bulky and requireexpensive manufacturing techniques to produce the lens array accurately.

Anti-reflection coatings for optical components are also known. Thesework on the principle that all the light incident normal to atransparent surface of refractive index n₁ is transmitted if the surfaceis covered with a thin film of refractive index n₂ which satisfies therelations:

    n.sub.0 ×n.sub.1 =n.sub.2.sup.2 and n.sub.2 ×d=λ/4

where n₀ is the refractive index of air; d is the cell thickness; and λis the wavelength of the light used. At other angles of incidence (andat other wavelengths) some of the light is reflected.

The anti-reflection coating only works optimally for light at normalincidence at a specific wavelength. However, by using a plurality ofdielectric layers of varying refractive index and thickness the spectralpass band at normal incidence can be improved. The invention is based onthe recognition that thin dielectric coatings can be put to novel use indisplays.

Collimator-like devices based on dielectric coatings are also known. Forexample, JP 07-43528 (Fuji Photo Film Co. Ltd.) discloses a filterdevice designed to protect a photo-sensitive material from lightentering at angles other than a specified angle and wavelength and JP59-67503 (Matsushita Electric Industrial Co. Ltd.) describes aspectroscopic dispersion element which uses wedge-shaped interferencefilters to pass only light perpendicular to the plane of the filter,further spectrally narrowing the passed light as a consequence of thespectral dispersion of the elements of the filter. U.S. Pat. No.5,572,500 (Pioneer) uses an interference filter to eliminate diffractivelobes to make a smaller beam spot in a laser recording system.

According to a first aspect of the invention there is provided acollimator for light in a narrow band around a predetermined wavelength,comprising an interference filter having a stack of dielectric layerswith optical thicknesses adapted to transmit light in the band incidentat a predetermined range of angles of incidence centred at normalincidence more efficiently than light in the band incident at otherangles, in which the interference filter has the form of a low-passfilter with a cut-off wavelength longer than the predeterminedwavelength.

In an alternative aspect the collimator comprises an interference filterhaving a stack of dielectric layers with optical thicknesses adapted totransmit light in the band incident at a predetermined range of anglesof incidence excluding a range centred at normal incidence moreefficiently than light in the band incident at angles around normalincidence, in which the interference filter has the form of a high-passfilter with a cut-off wavelength longer than the predeterminedwavelength.

The invention is also directed to light sources and modulating devicessuch as displays using a collimator for light in a band around apredetermined wavelength, comprising an interference filter having astack of dielectric layers with optical thicknesses adapted to transmitlight in the band incident over a predetermined range of angles ofincidence more efficiently than light in the band incident at otherangles.

The collimator can be used in combination with a diffuse source toprovide a light input for an optical modulating device, in particular anLCD. An interference filter is particularly advantageous since the thinfilm takes virtually no space and can be incorporated as a layer on anexisting component such as a glass plate for the LCD.

The filter is of course suited for use with a narrow range ofwavelengths of the input light. Hence the light source should benear-monochromatic, or narrow-band, for best results. This would notnormally be desirable for LCDs, at least for colour displays, but forPLLCDs it is usually the case that the input is narrow-band near-visibleUV light, which is ideally suited for use with a sharp cut-off filter.The colour output is then provided by the phosphors.

In either the angular or wavelength domain a general filter can beconsidered as one of 3 types: a low-pass filter, a high-pass filter, ora bandpass filter. When considered in the wavelength domain the low-passfilter is also termed a short-wave pass filter, similarly the high-passfilter is termed a long-wave pass filter. The short-wave and long-wavefilters can also be referred to as edge filters, having a single edge intheir response; the position of this edge is itself referred to as thecut-off of the filter. For clarity the cut-off of such a filter isgenerally defined as the point at which the response (usually thetransmission) is 50% at normal incidence. A similar definition can beused for the cut-off of a filter in the angular domain. Idealtransmission-wavelength graphs for these filter types are shown in FIG.1.

Bandpass filters have two edges and therefore two cut-offs; in thewavelength domain these are referred to as the long-and short-wavelengthcut-offs.

It is known that the form of the response of an interference filter, inthe wavelength domain, moves to shorter wavelengths as the angle ofincidence increases; this is shown diagrammatically in FIG. 2.

If the performance of a filter in the wavelength domain at normalincidence has a bandpass-type appearance then the performance in theangular domain at a particular wavelength can be predicted to a greateror lesser extent using this quality. For example, and referring to FIG.3, the performance in the angular domain at wavelength λ₀ will be thatshown in FIG. 4 (referred to as a low-pass filter) whilst theperformance at λ₁ will be that shown in FIG. 5 (a bandpass filter).

These figures are only schematic, of course, because strictly speakingthe form of the response is not preserved exactly as the angle ofincidence increases. FIG. 6 shows the actual response for a short-wavepass edge filter prepared for use in the invention at two angles ofincidence, normal and 30°.

Nevertheless it can be seen that for wavelengths close to the cut-offthe pass characteristics are sensitive to the angle of incidence. Awavelength of, say, 390 nm will be largely passed at normal incidencebut nearly totally reflected at 30°.

In PLLCD applications, the required performance in the angular domain isnormally that of the low-pass filter. In this case the required responsein the wavelength domain need not be precisely "bandpass" and ashort-wave edge filter, such as shown in FIG. 4, may well be adequate.

If near-normal light is to be passed then the cut-off of the edge filtershould be placed on the long-wavelength side of the design wavelength(i.e. of the peak of the spectrum of the source). For example thecut-off for the filter of FIG. 6 is at 396 nm which is on thelong-wavelength side of a design wavelength such as 388 nm. Theresulting response, in the angular domain at 388 nm, is shown in FIG. 7.This response shows that the angular distribution of a 388 nm diffusesource would be restricted to ±25° or so upon passing through thefilter.

A tighter distribution would be achieved by placing the cut-off closerto the design wavelength, and clearly a trade-off takes place:increasing the cut-off wavelength maximises the transmission of light,but the light will have a wider angular divergence. On the other hand ifthe cut-off wavelength is lowered, so that the wavelength of themonochromatic light lies near or even on the steep edge at the limit ofthe pass band of the graph of transmission coefficient againstwavelength, then improved collimation results at the cost of lowerbrightness.

For practical filters such as are shown in FIG. 6 the steep edgeterminates at the top in a smaller or larger peak or "first maximum"whose size depends on the quality of the filter. This peak is useful fordefining the wavelength of the source in relation to the filtercharacteristics. In general the predetermined wavelength should be closeto the cutoff, on its short wavelength side, to allow sufficientthroughput of light. The layers could be adapted to ensure that thetransmission at the predetermined wavelength is at least 80 or 90% ofthe transmission at the first maximum. As an alternative criterion, ifthe light source has a finite bandwidth Δ (as in practice it will) thenthe cutoff of the filter should be designed to ensure that substantiallyall (at least 90%) of the light output is passed (not reflected) by thefilter.

In a variant embodiment, if only off-normal light should be transmitted,a high-pass filter can be used, with the source wavelength (i.e. thepredetermined wavelength) lying on the short-wavelength side of thecut-off.

The substrate is preferably a flat plate and the collimator is used incombination with an optical device which makes use of the collimatedlight output.

The efficiency of a simple stack can be quite low, for example an ideallow-pass angular filter with the cut-off at 30°, would have anefficiency of 50% when used with a Lambertian source. In order toimprove the efficiency or overall throughput a means of regenerating therejected light is provided; this is possible because light that is notpassed by the filter is not absorbed but, in the case where the filteris being used in transmission, reflected. The rejected or reflectedlight is then re-presented to the filter by a suitable means,exemplified by a highly reflecting but diffusing mirror (see FIG. 8).

Since there are many parameters that can be varied to adjust thetransmission spectrum the easiest procedure to design a collimatoraccording to the invention is to use a computer program which cancalculate the transmission coefficient as a function of wavelength andangle for a plurality of layers of given dielectric constants. Such acomputer program is not difficult to write.

The two dielectric materials are preferably chosen to have refractiveindices, at the predetermined wavelength, as widely spaced as possible.For example, tantalum pentoxide (Ta₂ O₅) and silicon dioxide (SiO₂) maybe chosen, which have refractive indices of about 2.3 and 1.47respectively at a wavelength of 388 nm. Titanium dioxide (TiO₂) would bean alternative (if somewhat lossy) higher-refractive-index material, andmagnesium difluoride an alternative lower refractive index material.

Once the materials are selected their thicknesses must be chosen. Asuitable program can then be used to evaluate the performance of theresulting filter. Good results have been obtained by selecting the highrefractive index layers to have identical thicknesses of a quarter ofthe predetermined wavelength, for instance.

Terminology such as (HL)^(n) will be used in which H refers to a layerof high-index material and L refers to a layer of low-index material.The thickness of each layer is considered to be one quarter wavelengthin the appropriate medium. A regular stack can be created by repeatingthe layers within the brackets `n`, times; hence a (HL)¹⁰ design willhave 20 layers, 10 of each material. Furthermore, arbitrary designs canbe described with this terminology, such as (HL)³ (2H)(LH)³ for example.See James D Rancourt, `Optical Thin Films--Users' Handbook` for furtherexplanation and examples.

The preferred embodiment of a short-wave-pass filter such as can be usedto restrict the angular distribution of light as described above isdesigned by designing an (HL)^(n) filter positioned correctly in thewavelength domain and then using an optimisation algorithm, e.g. asprovided in TFCalc, to improve the performance against appropriateoptimization targets. This process is shown in FIG. 9 and FIG. 10. Ifthe pass band in the wavelength domain has been positioned correctly bythe above process then the required performance in the angular domainwill be achieved. FIG. 9 shows the un-optimized edge filter constructedfrom a (HL)¹³ stack, whilst FIG. 10 shows the edge filter afteroptimization.

The collimator according to the invention has the apparent disadvantagethat it is only suitable for light in a wavelength band around thepredetermined wavelength. However, for applications using substantiallymonochromatic light this is not a disadvantage and the greatersimplicity, ease of manufacture and reduced thickness of the collimatoraccording to the invention as compared to multiple lens arrays is highlyadvantageous. The collimator is therefore preferably used in combinationwith a source of matched substantially monochromatic light.

Preferably, the layers are inorganic layers, which can be depositedusing known techniques. Preferably, no more than two differentdielectric materials are used provided alternately on the substratesurface. In order to achieve sufficient angular discrimination, it isgenerally necessary to provide at least three pairs of dielectriclayers. In some current embodiments, around twenty-five pairs are used,which provide good angular discrimination without creating excessivemanufacturing difficulties.

For PLLCDs the predetermined wavelength is preferably about 388 nm, but365 nm (a common wavelength for mercury discharge lamps) can also beused. The dielectrics should have refractive indices as widely separatedas possible. A predetermined wavelength of 405 nm is possible inalternative embodiments. It is also conceivable to select the wavelengthfor a given stack, rather than to adapt the stack for a givenwavelength. However, since most light sources emit at a fixed wavelengthit is generally easier to adjust the thicknesses of the layers.

The invention can be used to collimate light for a liquid-crystaldisplay, or at least to improve the collimation of light in such adisplay. Such a liquid-crystal display can include a liquid-crystal cellfor modulating monochromatic light sandwiched between a front and a reartransparent substrate, in which a stack of layers with differentdielectric constants is provided on the rear transparent substrateadapted to transmit light in a predetermined narrow-wavelength bandincident substantially normally to the substrate and to reflect suchlight incident at other angles, e.g. above about 25°.

Preferably, the liquid-crystal display is a photoluminescentliquid-crystal display (PL-LCD) in which a photoluminescent layer isprovided at the front transparent substrate. The liquid-crystal displaymay be of the type having two polarizers sandwiching the liquid-crystalcell, one provided on the outside of each of the transparent substrates.The polarizers could be either linear or circular, and for some types ofliquid-crystal cell one of each is used. The stack is preferably placedon (i.e. on the light-source side of) the first polarizer.

Preferably, the liquid-crystal display further comprises a light sourcearranged behind the rear transparent substrate to produce thenarrow-band input radiation.

Most of the light emitted by the light source is not transmitted by theplurality of layers and is specularly reflected from them. Therefore, inorder not to waste this reflected light, a diffuse reflector ispreferably provided behind the light source, as mentioned above. Some ofthe light reflected by the diffuse reflector will be within the desiredangular pass band and will be transmitted by the plurality of layers;the rest of the light will be reflected once again back to the diffusereflector for further reflection back towards the LCD.

In edge-lit embodiments, the light source may be provided at the side ofthe transparent substrate and a scattering surface provided on theopposite side of the transparent substrate to the stack of layers.

For a better understanding of the invention a specific embodiment willnow be described, purely by way of example, with reference to theaccompanying figures, in which:

FIG. 1 shows schematic transmission coefficients of ideal edge andband-pass filters,

FIG. 2 shows the shifted response of a band-pass filter with a varyingangle of incidence,

FIG. 3 shows an ideal band-pass filter in the wavelength domain,

FIG. 4 shows the incidence angle dependence of the filter shown in FIG.3 at a wavelength λ₀,

FIG. 5 shows the incidence angle dependence of the filter shown in FIG.3 at a wavelength λ₁,

FIG. 6 shows an actual filter response at two angles of incidence 0° and30°, for light of wavelength 388 nm,

FIG. 7 shows the incidence angle dependence of the filter of FIG. 6,

FIG. 8 shows a schematic arrangement according to the invention,

FIG. 9 shows the transmission of an unoptimized edge filter constructedfrom a (HL)¹³ stack,

FIG. 10 shows the edge filter after optimization,

FIG. 11 shows a schematic of a liquid-crystal display device accordingto the invention, and

FIG. 12 shows an alternative backlight arrangement.

In FIG. 11, a liquid crystal layer 1 is shown contained between a fronttransparent substrate 3 and a rear transparent substrate 5. Twopolarizers 7, 9 are provided between the liquid crystal cell and thetransparent substrates. Electrodes are provided on the transparentsubstrates 3, 5 to control the liquid crystal cell 1. In thisembodiment, the transparent substrates 3, 5 are made of glass. Adielectric stack 11 is provided on the rear face of the rear transparentsubstrate 5. A diffuse reflector 15 is provided behind the rear of thedisplay, and a light source 13 is provided between the diffuse reflector15 and the dielectric stack 11. The light source can be, for instance, aset of phosphor-coated mercury vapour discharge tubes, emitting UV lightat about 365 or 388 nm. RGB Phosphors 17 are provided on the fronttransparent substrate 3 to provide the visible image when struck by UVlight passed by the liquid crystal.

The dielectric stack 11 comprises a stack of pairs of dielectrics. Thechosen dielectrics in many embodiments are Ta₂ O₅ and SiO₂ whoserefractive indices are n=2.3 and n=1.47 respectively at a wavelength λof 388 nm. In a first embodiment the thickness of the layers with thelower dielectric constant is given by λ/4, here 97 nm, and the layerswith the higher dielectric constant have a thickness of λ/2.955, here131 nm (or 91 and 124 nm respectively for light at 365 nm). In a moresophisticated embodiment the thickness of each layer is chosenindependently, and twenty-five pairs of layers make up the dielectricstack 11, i.e. fifty layers are provided, twenty-five each of Ta₂ O₅,and SiO₂ in an alternating arrangement. The thicknesses are as given inthe table at the end of the description. In an alternative embodimentpairs of layers of titanium dioxide and magnesium difluoride are used.

FIG. 6 shows the transmission coefficient T of the dielectric stack usedin the embodiment of FIG. 11 as a function of wavelength. As can beseen, maximum reflection of the dielectric stack occurs for wavelengthsbetween 400 nm and 480 nm and maximum transmission occurs below about385 nm. A suitable light source is a low-pressure mercury discharge lampwith a phosphor which emits light around a wavelength of 365 nm in awavelength band of width approximately 20 nm; more recently phosphorsemitting at around 388 nm have been used, with an advantageously narrowFWHM bandwidth of 13-14 nm--see for instance L Ozawa and H N Hersch,Journal of the Electrochemical Society, Vol 122 No 9, p. 1222 (1975).

The transmission coefficient of the stack as a function of incidentangle at 388 nm is shown in FIG. 7. As can be seen, transmission forlight 20° away from normal is much less than for normally incidentlight, and very little light incident at 30° from the normal istransmitted. Whether such collimation is sufficient depends on the pixelsize and exact structure of the display, and on the opto-electronicproperties of the liquid crystal. In any event, the collimation is animprovement. Such collimation is particularly important for colourPL-LCD displays in which it is important that light hits the phosphorhaving the correct colour, and not a neighbouring phosphor, whichgenerally has a different colour.

In use, the light source 13 emits light (shown in FIG. 11 using arrows).The source is shown schematically as a point but in practice would beone or more tubes. Part of the light is emitted in the correctdirection, is normally incident on the dielectric stack and passesthrough it. The remainder of the light is reflected back to the diffusereflector 15. This reflector returns the light to the dielectric stackat various angles; once again some will pass through the dielectricstack and most will be reflected, repeating the process.

The dielectric stack transmits only a small proportion of the incidentlight. Therefore, it is very important that the diffuse reflector has agood reflectivity in order to avoid wasting too much of the emittedlight.

The light incident upon the dielectric stack at a substantially normalangle of incidence is transmitted through to the liquid crystal cell 1.The liquid crystal 1, in conjunction with the polarizers 3, 5, modulatesthe light. The phosphors 17 of the photoluminescent screen emit visiblelight to form the image where they are excited by light transmittedthrough the polarizers 3, 5 and the liquid crystal cell. In the regionwhere no light is transmitted through the polarizers 3, 5 and cell 1,the phosphors 17 remain dark.

The dielectric stack in this embodiment has another advantage whencombined with a liquid crystal display using phosphors and ultravioletlight (i.e. light of a wavelength less than about 400 nm): it filtersout the visible lines of a mercury lamp, at least for normal incidence,at wavelengths between 400 and 480 nm, as can be seen from FIG. 6. Thepeaks of transmission above 480 nm belong to the 30° curve.

The collimator of the present invention can be combined with othercollimating components if desired. Furthermore, it is possible to mountthe dielectric stack on other components of a liquid-crystal displaysystem and not directly onto the transparent substrate surrounding theliquid-crystal cell. Indeed, the collimator is useful wherever roughcollimation of highly monochromatic light is required and notnecessarily only for displays.

FIG. 12 shows an alternative backlight arrangement. Light sources 13,e.g. phosphor-coated low pressure mercury discharge tubes with suitablereflectors as shown, are provided around the edges of a flat transparentsubstrate 19; a diffuse reflector 15 is provided on the rear of thetransparent substrate and the dielectric stack 11 on the front surface.The arrangement operates in the same way as the arrangement of FIG. 11,except that the light source is no longer between the diffuse reflectorand the stack.

                  TABLE                                                           ______________________________________                                                      /nm             /nm                                             ______________________________________                                        1st pair    Ta.sub.2 O.sub.5                                                                      64.55      SiO.sub.2                                                                          85.06                                     2nd pair    Ta.sub.2 O.sub.5                                                                      53.18      SiO.sub.2                                                                          78.83                                     3rd pair    Ta.sub.2 O.sub.5                                                                      50.06      SiO.sub.2                                                                          79.53                                     4th pair    Ta.sub.2 O.sub.5                                                                      52.17      SiO.sub.2                                                                          76.02                                     5th pair    Ta.sub.2 O.sub.5                                                                      50.66      SiO.sub.2                                                                          76.08                                     6th pair    Ta.sub.2 O.sub.5                                                                      49.86      SiO.sub.2                                                                          78.21                                     7th pair    Ta.sub.2 O.sub.5                                                                      51.33      SiO.sub.2                                                                          74.10                                     8th pair    Ta.sub.2 O.sub.5                                                                      49.98      SiO.sub.2                                                                          75.34                                     9th pair    Ta.sub.2 O.sub.5                                                                      51.15      SiO.sub.2                                                                          77.02                                     10th pair   Ta.sub.2 O.sub.5                                                                      50.03      SiO.sub.2                                                                          77.47                                     11th pair   Ta.sub.2 O.sub.5                                                                      49.16      SiO.sub.2                                                                          76.33                                     12th pair   Ta.sub.2 O.sub.5                                                                      48.56      SiO.sub.2                                                                          77.33                                     13th pair   Ta.sub.2 O.sub.5                                                                      50.66      SiO.sub.2                                                                          77.64                                     14th pair   Ta.sub.2 O.sub.5                                                                      48.62      SiO.sub.2                                                                          76.33                                     15th pair   Ta.sub.2 O.sub.5                                                                      50.08      SiO.sub.2                                                                          78.62                                     16th pair   Ta.sub.2 O.sub.5                                                                      49.88      SiO.sub.2                                                                          76.80                                     17th pair   Ta.sub.2 O.sub.5                                                                      49.70      SiO.sub.2                                                                          75.73                                     18th pair   Ta.sub.2 O.sub.5                                                                      51.11      SiO.sub.2                                                                          76.91                                     19th pair   Ta.sub.2 O.sub.5                                                                      51.40      SiO.sub.2                                                                          78.40                                     20th pair   Ta.sub.2 O.sub.5                                                                      47.41      SiO.sub.2                                                                          81.94                                     21th pair   Ta.sub.2 O.sub.5                                                                      54.55      SiO.sub.2                                                                          77.57                                     22th pair   Ta.sub.2 O.sub.5                                                                      56.02      SiO.sub.2                                                                          91.77                                     23th pair   Ta.sub.2 O.sub.5                                                                      60.17      SiO.sub.2                                                                          95.25                                     24th pair   Ta.sub.2 O.sub.5                                                                      59.75      SiO.sub.2                                                                          82.80                                     25th pair   Ta.sub.2 O.sub.5                                                                      58.70      SiO.sub.2                                                                          38.31                                     ______________________________________                                    

What is claimed is:
 1. A collimator for light in a narrow band around apredetermined wavelength, comprising an interference filter (11) havinga stack of dielectric layers with optical thicknesses adapted totransmit light in the band incident at a predetermined range of anglesof incidence centered at normal incidence more efficiently than light inthe band incident at other angles, in which the interference filter hasthe form of a low-pass filter with a cut-off wavelength longer than thepredetermined wavelength.
 2. A collimator according to claim 1, in whichthe pass band of the filter is optimized so as to be substantially flat.3. A collimator according to claim 1, in which the response of thefilter has a first peak of transmission at the short-wavelength side ofthe cut-off at a wavelength which is less than or equal to thepredetermined wavelength.
 4. A collimator according to claim 1, whereinthe predetermined wavelength falls in the visible region.
 5. Acollimator according to claim 1, wherein the predetermined wavelengthfalls in the ultra-violet.
 6. A directional light source including acollimator according to claim 1 and a diffuse lighting arrangementadapted to produce light in a narrow band around the predeterminedwavelength.
 7. A light source according to claim 6, further having ameans to return light rejected by the collimator to the collimator toimprove the light throughput.
 8. A light source according to claim 7, inwhich the light return means includes a highly reflecting diffusesurface.
 9. A light modulator comprising an optical modulating devicefor light in a narrow band around a predetermined wavelength and acollimator according to claim
 1. 10. A liquid-crystal display,includinga collimator (11) according to claim 1 and a liquid-crystalmodulator (1) for modulating light.
 11. A liquid-crystal displayaccording to claim 10, further including a photoluminescent layer (17)for receiving the modulated light and producing a display.
 12. Aliquid-crystal display according to claim 11, wherein the interferencefilter (11) is formed directly on one face (5) of the liquid-crystalmodulator and the photoluminescent layer is formed on the opposite face(3).
 13. A collimator for light in a narrow band around a predeterminedwavelength, comprising an interference filter (11) having a stack ofdielectric layers with optical thicknesses adapted to transmit light inthe band incident at a predetermined range of angles of incidenceexcluding a range centered at normal incidence more efficiently thanlight in the band incident at angles around normal incidence, in whichthe interference filter has the form of a high-pass filter with acut-off wavelength longer than the predetermined wavelength.
 14. Adirectional light source including:a collimator for light in a narrowband around a predetermined wavelength, comprising an interferencefilter (11) having a stack of dielectric layers with optical thicknessesadapted to transmit light in the band incident at a predetermined rangeof angles of incidence centered at normal incidence more efficientlythan light in the band incident at other angles; and a diffuse lightingarrangement adapted to produce light in a narrow band around thepredetermined wavelength, for passage through the collimator.
 15. Alight modulator comprising an optical modulating device for light in anarrow band around a predetermined wavelength and a collimator for lightin a narrow band around a predetermined wavelength, the collimatorcomprising an interference filter (11) having a stack of dielectriclayers with optical thicknesses adapted to transmit light in the bandincident at a predetermined range of angles of incidence centered atnormal incidence more efficiently than light in the band incident atother angles; in which the pre-determined range of angles is such as toimprove the performance of the light modulator.
 16. A light modulatoraccording to claim 15 in which the modulating device is a liquid-crystaldevice.
 17. A display device including a light modulator according toclaim
 15. 18. A method for collimating light from a diffuse extendedlight source which emits in a narrow band around a pre-determinedwavelength, in which the said light is incident upon and is transmittedthrough a collimator comprising an interference filter (11) having astack of dielectric layers with optical thicknesses adapted to transmitlight in the said narrow band incident at a predetermined range ofangles of incidence centered at normal incidence more efficiently thanlight in the band incident at other angles.